This invention relates to an air-fuel ratio detecting apparatus which detects the air-fuel ratio of the intake mixture sucked into an engine from time to time by sensing the oxygen concentration of its exhaust gas.
Among various means proposed for eliminating noxious exhaust pollutants such as hydrocarbons, and oxides or carbon and nitrogen (HC, CO and NO.sub.x) is a three way catalytic converter which can eliminate HC, CO and NO.sub.x at the same time. For the simultaneous treatment of these three noxious combustion products, the three way catalytic converter necessitates exact control of the intake mixture to the stoichiometone air-fuel ratio, which in turn requires an accurate air-fuel ratio control system.
There is a known air-fuel ratio detecting apparatus taking advantage of the known fact that electromotive force is produced if an oxygen partial pressure exists between two electrodes in an electrolyte having oxygen ions. Based on the principle that oxygen concentration in exhaust gas changes with the intake air-fuel ratio, this air-fuel ratio detecting apparatus uses a solid electrolyte and keeps one electrode exposed to the atmosphere and the other electrode in contact with the exhaust gas. Usually platinum is used as the electrodes. The catalytic action of platinum lowers the oxygen partial pressure at the electrolyte surface substantially to the equilibrium concentration, as a result of which electromotive force changes abruptly in the vicinity of the theoretical air-fuel ratio.
The known air-fuel ratio detecting apparatus of the above-described type have had several shortcomings. For instance, phosphorous and lead (P and Pb) in exhaust gases adhere to platinum to ruin its catalytic activity. Impingement of small iron pieces and other fine grains floating in exhaust gases scratches off the electrolyte or causes separation of the electrodes, which leads to malfunction of the detecting apparatus.